Suppression of a Parasitic Pump Side-Scattering in

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• Suppression of a Parasitic Pump Side-Scattering
  in
• Backward Raman Amplifiers of Laser Pulses in
  Plasmas
• A. Solodov, V. M. Malkin, N. J. Fisch

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Abstract
In backward Raman amplifiers (BRA), the pump
laser pulse can be prematurely depleted through
Raman scattering, seeded by the plasma noise, as
the pump encounters plasma before reaching the
counter-propagating seed pulse. It was shown
previously that detuning of the Raman resonance,
either by a plasma density gradient or a pump
frequency chirp, can prevent the premature pump
backscattering, even while the desired
amplification of the seed pulse persists with a
high efficiency. However, parasitic pump
side-scattering is not automatically suppressed
together with the parasitic backscattering, and
might be even more dangerous for BRA. What we
show here is that by combining the above two
detuning mechanisms one can suppress parasitic
pump side-scattering as well. Apart from the
simplest counterpropagating geometry, we examine
BRA for arbitrary angles between the directions
of pump and seed propagation. We show that, by
selecting an appropriate direction of the plasma
density gradient, one can favorably minimize the
detuning in the direction of the seed pulse
propagation, while strongly suppressing the
parasitic pump side-scattering in all the other
directions. This work was supported in part by
DOE and DARPA.
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Conceptual Scheme of Backward Raman Amplifiers
(BRA)
kpk1-k2, ?p?1-?2
The plasma wave forms a 3D Bragg cell grating
that scatters power from the pump into the seed
At the nonlinear amplification stage the
amplified seed can completely deplete the pump
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Conceptual Scheme of BRA (Continued)
Resent studies showed that short pumped pulses of
nearly relativistic non-focused intensities are
expected I1017 W/cm2 for ?1 µm 1. This is 5
orders of magnitude higher than currently
available through chirped pulse amplification
2. Additional intensity gain is provided by
focusing. 1 V. M. Malkin, G. Shvets, and N. J.
Fisch, Phys. Rev. Lett. 82, 4448 (1999). 2 G.
A. Mourou, C. P. J Barty, and M. D. Perry, Phys.
Today 51, 22 (1998).
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Problem Pump Can be Prematurely Depleted by
Raman Scattering Seeded by Thermal Langmuir
Noise, before it Reaches the Seed Pulse
It is the same efficiency of stimulated Raman
scattering, that makes possible the fast
compression, which can complicate the pump
transporting to the seed. The problem is
aggravated by the fact that the linear Raman
scattering (responsible for the noise
amplification) has a larger growth rate than its
nonlinear counterpart (responsible for the useful
amplification of the seed).
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The Premature Pump Backscattering Can be
Suppressed by an External Detuning of the Raman
Resonance 3
d?d?plasma-d?pump It appears to be possible to
suppress the unwanted Raman backscattering of the
pump by noise, while not suppressing the
desirable seed pulse amplification. The
filtering effect occurs because in the nonlinear
regime the pumped pulse duration decreases
inversely proportional to the pulse amplitude.
The increased frequency bandwidth allows to
tolerate larger and larger external frequency
detuning.
3 V. M. Malkin, G. Shvets, and N. J. Fisch,
Phys. Rev. Lett. 84, 1208 (2000).
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The Goal of the Present Paper is to Analyze How a
Parasitic Pump Side-Scattering in BRA Can be
Suppressed
The Raman growth rate maximizes for
backscattering
(linearly polarized pump with normalized
amplitude a0eA/mc2, scattering in the plane
perpendicular to the pump polarization). However,
the side-scattered radiation has more time and a
longer distance to be amplified by the pump,
before it leaves the plasma. These two effects
practically compensate each other making
suppression of pump side-scattering an important
task.
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Main Equations
The linear stage of SRS is described by where
the vector-potential envelopes of the pump and
scattered Stokes waves and Langmuir wave electric
field are defined The pump wave frequency,
wavenumber, and the unit polarization vector are
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Main Equations (Continued)
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Main Equations (Continued)
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Greens Function
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Greens Function (Continued)
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Premature Pump Backscattering and Side-Scattering
in Absence of Detuning
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Suppression of a Parasitic Pump Side-Scattering
in a Backward Raman Amplifier
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Suppression of a Parasitic Pump Side-Scattering
in a Backward Raman Amplifier (Continued)
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Suppression of a Parasitic Pump Side-Scattering
in a Backward Raman Amplifier (Continued)
circular polarization also linear
polarization, fp/4
linear polarization, fp/2
linear polarization, fp/6
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Scattering Cross-Section of the Pump Beam in BRA
with Frequency Detuning
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Scattering Cross-Section of the Pump Beam in BRA
with Frequency Detuning (Continued)
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Suppression of a Parasitic Pump Scattering in a
Raman Amplifier with Arbitrary Angles between
Pump and Seed Pulses
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Suppression of a Parasitic Pump Scattering in a
Raman Amplifier with Arbitrary Angles between
Pump and Seed Pulses (Continued)
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Suppression of a Parasitic Pump Scattering in a
Raman Amplifier with Arbitrary Angles between
Pump and Seed Pulses (Continued)
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Suppression of a Parasitic Pump Scattering in a
Raman Amplifier with Arbitrary Angles between
Pump and Seed Pulses (Continued)
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Conclusions

• Detuning of the Raman resonance by a plasma
  density gradient and a pump chirp can suppress
  parasitic pump side-scattering in Backward Raman
  Amplifiers
• By selecting appropriate values of the pump
  chirp and plasma density gradient as well as
  direction of the density gradient, one can
  favorably minimize the detuning in the direction
  of the seed pulse propagation, while strongly
  suppressing the parasitic pump side-scattering in
  all the other directions